1. Field of the Invention
The present invention relates generally to integrated circuits used to drive liquid crystal displays (LCDs), and more particularly to an integrated circuit for economically driving an LCD display using column inversion and/or pixel inversion techniques.
2. Description of the Related Art
The trend toward larger, higher resolution, higher color displays in notebook computers is forcing display manufacturers to use new electrical drive methods within the integrated circuits which drive the displays. Thin Film Transistor (TFT) displays for notebook computers have quickly migrated from eight inch, 256 color, low-resolution displays to 12.1 inch, 262,000 color, high-resolution displays. In addition, the emerging cathode-ray-tube (CRT) replacement market is promising 16-inch, 16.7 million color, very high resolution LCD displays in the near future. Current methods used to drive these displays are subject to excessive power dissipation and reduced image quality at resolutions above "Super VGA".
LCD display panel manufacturers are returning to Direct Drive as the answer to these problems. Direct Drive was originally used several years ago by many major LCD manufacturers; however, Direct Drive was later abandoned due to cost concerns. Direct Drive requires higher-voltage driver circuits (i.e., driver circuits which generate a larger range of analog output voltages) which, historically, have been much more expensive to produce; one reason for this greater expense is that higher voltage ranges typically require larger device geometries, and more chip real estate. Yet, Direct Drive offers dramatic improvements in image quality and power dissipation, as compared with current methods used to drive complex displays.
The "complexity" of a display is a combination of the display size, display resolution, and number of colors. As display complexity increases, power dissipation associated with such display typically increases. In addition, as display complexity increases, the quality of the image displayed typically tends to decrease. The problems associated with power dissipation and image quality are leading display manufacturers back to Direct Drive techniques for driving Flat Panel LCD Displays.
A typical TFT display is made up of both rows and columns. The intersection of each row and column represents the location of a TFT color cell, called a pixel. The circuitry for driving such display includes integrated circuits known as row drivers which control each row of a display, and simply turn each row on or off, one at a time, to allow access to the pixels in that row. The circuitry used to drive the LCD display also includes integrated circuits known as column drivers which are responsible for updating the shade of color in the pixels of the selected row. The present invention is directed to such column driver integrated circuits.
To produce color shades, the pixels in an LCD display require an alternating voltage, which alternates between "positive" and "negative" polarities. In addition, the magnitude of such voltage within the "positive" or "negative" range, will determine the shade of the color, such as ranging from white to black, or from light blue to dark blue.
The aforementioned term "Direct Drive" refers to the ability of the column driver chips to directly provide the alternating voltage and the variable magnitudes of such voltage to each pixel cell. Other driving methods rely upon additional integrated circuits in the system to create alternating polarities. For example, it is presently typical to apply an alternating voltage to the backplane of an LCD display while applying voltages of opposite polarity to each of the columns within the LCD display. The column driver circuits in such common backplane systems supply only the variable voltage magnitude, while additional circuitry must drive the common backplane to alternate the voltage across each pixel; this technique is called V-com Modulation because of the additional integrated circuits used to modulate the positive and negative voltages on the common plate, or backplane of the display Thus, Direct Drive is capable of forcing both polarity and magnitude on the pixels by driving the columns only, while V-com Modulation requires an additional polarity driver to drive the large common plate of the display. For the reasons explained below, driving the large common plate using V-com Modulation increases power dissipation and reduces the image quality of the display.
The various techniques used by display manufacturers to alternate the voltages applied to the pixels are referred to as inversion methods. In a rather straightforward technique called frame inversion, the entire display (i.e., all of the pixels in the display) is updated with various positive polarity voltages during a first frame, by negative polarity voltages in a second frame, by positive polarity voltages in a third frame, and so forth. In other words, all of the pixels in the LCD array are positive concurrently during one frame, and all of the pixels in the LCD array are negative concurrently in the next frame. Incidentally, it should be understood that the term "negative voltage" is a relative term and refers to the voltage difference between a pixel cell and the common terminal of the display. A pixel voltage can be considered "negative" if below +5 volts, for example, even though such voltage is above ground potential.
In a second technique known as row inversion, the polarity of the voltage applied to the pixels in successive, adjacent rows of the display is alternated; during a first frame period, the voltages applied to the first row of pixels are positive, the voltages applied to the second row of pixels are negative, the voltages applied to the third row of pixels are positive, etc. During a next succeeding frame period, this relationship is reversed, i.e., the voltages applied to the first row of pixels are negative, the voltages applied to the second row of pixels are positive, the voltages applied to the third row of pixels are negative, etc.
A third technique that has also been used is known as column inversion. As the name implies, during a first frame period, all of the pixels within a first column are at positive voltages, all of the pixels in the second column are at negative voltages, all of the pixels in the third column are at positive voltages, etc. During a next succeeding frame period, the relationship is reversed, i.e., all of the pixel voltages in the first column are negative, all of the pixel voltages in the second column are positive, all of the pixel voltages in the third column are negative, and so forth.
Finally, the method known as pixel inversion, causes each pixel located at a particular row and column to have a voltage that is opposite in polarity to the voltage of any adjacent pixel during any frame period. For example, during a first frame period, the pixel located at row 1, column 1, is positive; the pixel located at row 1, column 2, is negative; the pixel located at row 2, column 1, is negative; and the pixel located at row 2, column 2, is positive. During the next succeeding frame period, the polarities are reversed, such that the pixel voltage at row 1, column 1, is negative; the pixel voltage at row 1, column 2, is positive; the pixel voltage at row 2, column 1, is positive and the pixel voltage at row 2, column 2, is negative.
The above-described column inversion and pixel inversion driving methods can provide significant improvements in power dissipation and image quality over the other inversion methods. The Direct Drive technique for driving pixel voltages can provide any of the four inversion methods described above. In contrast, V-com Modulation can provide only frame inversion or row inversion, since positive and negative voltages are provided via the common plate or backplane. The use of such a common plate to provide the polarity of the pixel voltages requires that, as each row is updated, the polarity of the pixels in that row must be identical to each other. This necessarily excludes the column inversion and pixel inversion methods.
The problem of image quality has been mentioned above. One component of image quality is known as "flicker". Since the human eye is very adept at noticing fluctuations or changes in a visual image, a display must be updated at a rather fast rate to prevent noticeable flicker. Flicker is even more noticeable when the fluctuation is over a larger area. Column inversion reduces flicker as compared with frame and row inversion; pixel inversion even further reduces the problem of flicker as compared with column inversion. Only the so-called Direct Drive method of applying pixel voltages can be used to achieve column inversion and pixel inversion.
Another aspect of image quality is the problem of "crosstalk"; crosstalk refers to errors caused by the presence of similar voltage polarities at neighboring pixels. Crosstalk errors can be canceled by ensuring that neighboring pixels use opposite polarities. Such crosstalk errors are minimized when pixel inversion is used; once again, pixel inversion requires that the Direct Drive method of driving pixel voltages be used.
The inversion method and drive method used to drive the LCD display also influence the amount of power dissipated. While frame inversion conserves power, frame inversion is also subject to flicker and high levels of crosstalk. Column inversion conserves power very well, eliminates flicker, but is still subject to low levels of crosstalk. Pixel inversion also reduces power dissipation (though not as well as column inversion); moreover, pixel inversion is not subject to either flicker or crosstalk, thereby producing the best image quality. Once again, column inversion and pixel inversion require the Direct Drive technique for applying pixel voltages. It should therefore be apparent that the combination of Direct Drive and pixel inversion for driving an LCD display is the best technique for dealing with the problems of power dissipation and poor image quality.
As mentioned above, LCD display manufacturers abandoned Direct Drive in the past as a result of its requirement for more-expensive, higher voltage column drivers. These higher voltage column driver integrated circuits typically required special manufacturing methods and were not readily available in significant volumes. In addition, for the relatively small low resolution displays of the past, V-com Modulation techniques were adequate.
LCD color display panels in wide use today typically require an alternating voltage having a magnitude of approximately ten volts in order to drive each pixel in the display. When V-com Modulation is used, the column driver integrated circuits need to produce output voltages that range only between approximately zero and +5 volts; the remainder of the voltage difference applied across each pixel is created by varying the polarity of the common voltage applied to the backplane of the display. In contrast, the Direct Drive method of applying pixel voltages requires that the integrated circuit column drivers have outputs capable of driving through the full ten volt output swing (zero volts to +10 volts).
In the past, high voltage integrated circuit column drivers have commonly included a separate digital-to-analog converter for each output driver terminal of such integrated circuit. Moreover, if the full range of output voltages to be applied on each column included 256 different voltages, for example, then each of the separate digital-to-analog converters had to be capable of generating each of such 256 different full-range voltages. Since one such column driver integrated circuit may typically include as many as 384 output terminals, the number and complexity of the required digital-to-analog converter circuits is significant, and can quickly increase the overall complexity of such column driver integrated circuits. Greater complexity typically means lower yields and higher costs.
Accordingly, it is an object of the present invention to provide an improved integrated circuit column driver for driving the columns of an LCD display which is adapted to use the Direct Drive method of applying pixel voltages without requiring a separate full-voltage range digital-to-analog converter for each column output terminal.
Another object of the present invention is to provide such an improved integrated circuit column driver which directly drives each pixel voltage but which does not require that any single digital-to-analog converter circuit produce a full-range analog output voltage.
Still another object of the present invention is to provide such an improved integrated circuit column driver that is compatible with either of the above-described column inversion and pixel inversion driving methods in order to limit power dissipation and improve the image quality of the display by reducing flicker and crosstalk.
A further object of the present invention is to provide such a column driver integrated circuit of reduced complexity for achieving higher yields and lower costs.
These and other objects of the present invention will become more apparent to those skilled in the art as the description of the present invention proceeds.